The invention relates to a method for determining the viscosity of an operating liquid of an internal combustion engine.
A method of this kind is known from DE 195 18 776 A1 which describes a method for establishing the viscosity of an operating liquid on the basis of the changes in the level of the operating liquid over a period of time. In this known method, the viscosity of the engine oil is determined after having turned off an internal combustion engine by measuring the variation of the engine oil level over a period of time. The return of the engine oil to the sump after the engine has been turned off takes place with a time delay that depends on the viscosity. Apart from the variation of the level of the oil against time, the oil temperature is also a factor in the known method because the viscosity of the engine oil generally depends on its temperature.
Apart from these variables that are to be measured in a simple manner, the measurement of the viscosity in accordance with the known method is, however, also influenced by other variables. For instance, a high dynamic loading of the internal combustion engine over a period of time immediately before it is turned off can lead to foaming of the engine oil. The consequent slower change in level after turning the engine off results in corruption of the actual measurement. An objective measured variable for the dynamic loading of the internal combustion engine cannot, however, be determined in a simple manner.
Furthermore, when the internal combustion engine is turned off while the vehicle is in an inclined position, this also has an adverse effect on the return flow of the engine oil into the sump.
Another disadvantage is due to the fact that the electronic modules needed for the measurement still have to be actively operated for the required period of time after the internal combustion engine has been turned off.
The object of the invention is to specify a method for determining the viscosity of an operating liquid of an internal combustion engine by measuring the level of said liquid, whereby the method supplies a precise result with few measured variables and is performed while the internal combustion engine is in operation.
This object is solved by a method having the features according to the present invention as described and claimed herein.
In the method for determining the viscosity of an operating liquid of an internal combustion engine of a vehicle, initially a first signal is acquired corresponding to the variation against time of the level and then a second signal is acquired corresponding to the variation against time of the state of the vehicle that has an influence on the measured level of the operating liquid. The first and the second signals are then fed to a filter that allows the signals to be analyzed with respect to their dynamic response. This results in a third signal and a fourth signal being generated, where the third signal is the result of filtering the first signal and the fourth signal is the result of filtering the second signal. The fourth signal is then compared with a threshold value and a ratio relating the fourth signal to the third signal is formed according to the result of the comparison. The mean value over a multiple number of values of the ratio of the fourth to the third signal corresponds to the viscosity of the operating liquid.
The advantage of the invention is that sensor signals that are already available from sensor arrangements which have been tried out in practice and are in standard production can be combined with each other and from this the viscosity can be determined as an additional parameter. The necessary computation is performed by standard control devices.
In an advantageous form of embodiment of the method, the transverse acceleration of the vehicle is measured as the second signal. The continuous measuring signal of the oil level sensor is gated with the continuous signal of the transverse acceleration sensor. The result leads to a statement on the viscosity of the oil. Transverse accelerations are measured regularly for other systems such as for example, ESP.
Acquisition of the measured values of the first and second signals is performed preferably at a rate of approximately 1 to 1.5/s. This rate of measurement is the viscosity and small enough to avoid generating enough to provide reliable information relating to excessively large volumes of data.
For filtering the first or the second signal, preferably a floating standard deviation is formed from a first quantity of values. The third or fourth signals respectively form the output of the filter. The floating standard deviation uses relatively simple statistical methods to provide good information on the dynamic response of the signals.
The number of values for determining the floating standard deviation here is in the range from 5 to 100. A value of 15 has been found in a series of measurements to be particularly advantageous.
The generation of the mean value of the ratio of the fourth signal to the third signal is effected advantageously over approximately 50 to 300 values.
The invention will now be described with reference to the examples of embodiment and the figures.